(1) Field of the Invention
The present invention relates to a redrawing method. More particularly, the present invention relates to a method in which the thickness of a preliminarily drawn cup of a covered metal sheet is uniformly reduced by bending and elongating a side wall portion of a can barrel while drastically reducing the damage of a covering layer.
(2) Description of the Prior Art
The production of a seamless can barrel by subjecting a covered metal sheet to drawing and redrawing has been carried out from old in the can-manufacturing industry. At this drawing-redrawing forming, the metal is caused to make such a plastic flow that the size increases in the height direction of the can but the size decreases in the circumferential direction of the can barrel. Accordingly, in the can barrel obtained by the drawing-redrawing forming, the thickness of the side wall of the can barrel increases toward the upper portion from the lower portion, and the thickness is extremely large at the upper end (open end) of the side wall.
It also is known that at the drawing-redrawing forming, a deep-drawn cup having a small diameter is formed at a curvature corner part of a redrawing die and the side wall portion is bent and elongated to reduce the thickness of the side wall portion. In this case, the thickness of the side wall portion as a whole is reduced, but the upper portion is influenced by compression in the circumferential direction and the thickness of the upper portion tends to increase.
As the means for eliminating this disadvantage, Japanese Patent Application Laid-Open Specification No. 501442/81 proposes a method in which the side wall portion is bent and elongated at a curvature corner part of a redrawing die and then, the side wall portion is ironed at a front die stroke part, whereby the thickness of the side wall portion is uniformalized throughout the height direction of the can.
According to this conventional method, since the upper portion of the side wall where the thickness is increased by compression in the circumferential direction is ironed, the thickness of the entire side wall portion is uniformalized, but since the compression stress and ironing fence are applied to the upper portion of the side wall, the covering resin layer is considerably damaged or the adhesion of the covering resin layer to the metal sheet tends to decrease. Therefore, in a final canned product, such a problem as corrosion or erosion of the metal arises, and a swollen can is formed by generation of hydrogen or a leak can is formed by pitting.